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Forest carbon

The “carbon cycle” refers to the constant movement of carbon from the land and water through the atmosphere and living organisms. This cycle is fundamental to life on Earth.

Forests are a vital part of the carbon cycle, both storing and releasing this essential element in a dynamic process of growth, decay, disturbance and renewal. At a global scale, forests help maintain Earth’s carbon balance.

Over the past four decades, forests have moderated climate change by absorbing about one-quarter of the carbon emitted by human activities such as the burning of fossil fuels and the changing of land uses. Carbon uptake by forests reduces the rate at which carbon accumulates in the atmosphere and thus reduces the rate at which climate change occurs.

How well forests will continue to remove the proportion of carbon now being emitted by human activities will affect the future rate of carbon increase in the atmosphere.

Defining carbon balance

Earth’s carbon balance is calculated as the carbon emissions from human activities minus the carbon uptake by oceans and land systems. Since the industrial use of fossil fuels began, the net carbon balance has resulted in increases in the atmospheric CO2 concentration from 280 parts per million to over 390 parts per million.

As a major forest nation, Canada is working to understand how today’s changing climate will affect the global carbon balance, the health of the country’s ecosystems and the flow of goods and services provided to Canadian society.

Canada also has international reporting obligations. Under the United Nations Framework Convention on Climate Change, Canada must monitor and report greenhouse gas (GHG) emissions and changes in the carbon stock in its managed forests. This means tracking changes that result from forest growth, decomposition, disturbances (fire and insects), harvesting and land-use changes. Land-use changes include afforestation (that is, the creation of new forests where none exist) and deforestation (that is, the conversion of forests to non-forest land uses such as agriculture).

Forests as carbon sources and carbon sinks

A forest is considered to be a carbon source if it releases more carbon than it absorbs. Forest carbon is released when trees burn or when they decay after dying (as a result of old age or of fire, insect attack or other disturbance).

A forest is considered to be a carbon sink if it absorbs more carbon from the atmosphere than it releases. Carbon is absorbed from the atmosphere through photosynthesis. It then becomes deposited in forest biomass (that is, trunks, branches, roots and leaves), in dead organic matter (litter and dead wood) and in soils. This process of carbon absorption and deposition is known as carbon sequestration.

The net balance of all of these carbon exchanges determines whether a forest is a carbon source or sink. Yet, the carbon source/sink balance is as dynamic as it is complex.

For the past century, Canada's managed forests have been a significant carbon sink, steadily adding carbon to that already stored. In recent decades, however, the situation has reversed in some years: Canada’s forests have become carbon sources, releasing more carbon into the atmosphere than they are accumulating in any given year.

Several factors have contributed to this shift. The annual total area burned by wildland fires has increased substantially. Unprecedented insect outbreaks have occurred. And annual harvest rates have shifted dramatically in response to economic demand, increasing in the 1990s and decreasing sharply with the global economic recession.

The combination of these events and activities has resulted in Canada’s managed forest acting as a net carbon source in years when large areas are burned.

This information helps Canada meet its international obligations to report annually on GHG emissions and carbon stock changes in the country’s forests. The system can also be used to simulate future scenarios that scientists and policy analysts can then use to design and assess mitigation and adaptation strategies.

About 20 years ago, CFS scientists began developing a sophisticated model of Canada’s forest carbon budget. The Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) uses the best available information on forests and tree growth, obtained from resource management agencies across Canada. Information on forest characteristics (such as age class, species and growth rates) is combined with data on processes that drive disturbances (such as harvest, fires, insect infestations and land-use change).

The CFS Carbon Accounting Team continually improves the model, incorporating new scientific information. Environment Canada uses the model’s latest estimates in its annual GHG National Inventory Report, the main document Canada produces to meet its international reporting obligations.

The model, supporting documentation and tutorials are available free-of-charge on the Internet and are used widely in Canada and several other countries.

An eye on the future

How climate change will affect the carbon source/sink balance of Canada’s forests is being closely studied by CFS researchers. It appears that some natural disturbance regimes in the country’s forests are already being influenced by changes in climate. Two examples of this are the increases in the frequency and severity of fire and pest infestations (like the mountain pine beetle in central British Columbia and Alberta). In turn, these events are resulting in forests releasing greater amounts of carbon into the atmosphere.

How will climate change affect the future forest carbon balance?

Climate change is expected to have a profound impact on the carbon balance of Canada’s forests. The biggest short-term impacts will result from changes in disturbance regimes. For example, scientists at Natural Resources Canada’s Canadian Forest Service predict that the forest area annually burned in Canada is likely to double by the end of the century, resulting in large emissions of carbon. More frequent and longer lasting droughts are expected to contribute to this increase. Similarly, increases in the area and intensity of insect outbreaks are expected to cause carbon losses. Already, climate change, in the form of warmer winters, has contributed to the major infestation of the mountain pine beetle in British Columbia and its recent spread over the Rocky Mountains into Alberta. A key question is whether the insect will be able to expand its range through Canada’s boreal forests.

Some aspects of climate change, such as longer growing seasons or greater concentrations of carbon dioxide in the atmosphere, are expected to increase tree productivity (at least initially). A warmer, wetter climate may also enhance decomposition rates. Northern regions of Canada are expected to warm faster than more southerly areas, resulting in the melting of permafrost; this may release methane from frozen soils and initiate decomposition of previously frozen organic carbon.

Researchers at CFS predict that if this trend continues, the forest area annually affected by wildland fires could at least double in much of the country by the end of the century. More fires, in combination with longer, more frequent droughts and a greater incidence of insect outbreaks, will add to the atmospheric carbon emissions by forests.

The outcome of all these interconnected events is likely to be further acceleration of the feedback loop: more emissions will lead to accelerated climate change, which in turn will enhance the conditions that create more carbon-releasing disturbances in Canada’s forests.